The present invention provides a method and apparatus for treating a slurry of comminuted cellulosic fibrous material to remove undesirable material prior to introducing the slurry to a paper machine while minimizing fiber loss. This invention is particularly applicable to the treatment of fibrous slurries produced from recycled paper, especially pre- and post-consumer recycled papers containing undesirable ink and ash particles.
In normal practice, prior to introducing a slurry of cellulosic fibrous material to a paper machine the virgin or recycled pulp slurry must be--among other things--treated to remove contaminants and introduce brightening and conditioning agents. These treatments and processes are typically performed in the pulp mill. After the stock is prepared it is transferred to the paper machine stock processing and preparation system. The processing and preparation system receives its fiber furnish either from an adjacent integrated pulp mill, from a pulp purchased from a market pulp supplier, or from a recycled pulp mill. Typically, the fiber is introduced as a slurry of fiber and liquid from an adjacent pulp mill or is processed from bales of virgin or recycled pulp that must first be re-pulped to produce an aqueous slurry. Typically, conventional stock processing and preparation includes repulsing or thickening and various cleaning, screening, and refining equipment with which the pulp slurry is treated.
Recent environmental concerns about conserving our forest resources has led to both public demand and legislative mandates requiring that more recycled pulp be used in the manufacture of paper, especially in the manufacture of newsprint. Many states now require that newsprint contain at least 15% post-consumer recycled pulp, or higher. For example, as of 1999 the state of California requires that newsprint contain a minimum of at least 50% post consumer recycled pulp. This introduction of recycled pulp to the paper machine increases the load upon the paper machine stock preparation and approach system for removing undesirable contaminants from the furnish. The increased content of ink, stickies, dirt, and other contaminants in the recycled pulp should be removed as efficiently as possible prior to the paper machine, where it can cause runnability problems, efficiency reduction, a loss in brightness, a loss in strength of the resulting paper, and negatively affect the drainability of the slurry on the paper machine itself. Though typically some of these contaminants are removed during stock processing and preparation, some contaminants are passed to the paper machine approach system and cause operating problems and increased fiber loss in the approach system.
In conventional paper machine approach systems, the slurry of fibers is typically cleaned and screened to produce a rejects stream containing oversized particles, pin chips, sand, ink particles and other undesirable debris, and some fiber. This reject stream of undesirable material is typically discarded as unusable.
In many existing systems, rejects from the last cleaning system(s) are taken to the effluent treatment for an associated mill or related installation. In some cases, part of the rejects (or even all of the rejects) are returned to the beginning of the process without any treatment. This then makes a closed loop for undesired solids and contaminants.
In the most recent systems, the last stage of the cleaning system includes a fiber saving device, such as a FIBERMIZER device sold by Celleco, which doesn't change the physical properties of the solids but just have lower separation efficiencies and so concentrates or increases the contaminants in the cleaning process circulation, and in that way reduces fiber losses.
In all the above-described conventional existing technologies the reduction of the losses is done by throttling the reject flow. When the reject outlet flow is throttled, the separation efficiency of the undesired contaminants is also reduced and the only way for these solids is to go into the accepts, and thus the risk of runnability and quality problems is increased. In this invention, this fiber-containing stream is treated to recover the useable fiber. The present invention provides a method and apparatus for minimizing or removing these undesirable contaminants from the stock flow, typically as an adjunct to a conventional approach system, while minimizing the loss of valuable fiber. This is achieved by mechanically agitating the flow of rejected material to reduce pin chip and fiber flocs so that they can be recovered and reused. Though mechanical dispersion or deflaking is common in conventional recycled-fiber treatment, typically only the screening and cleaning accept flow is mechanically treated to improve ink dispersion and for deflaking. The screening and cleaning reject flow is typically untreated and discarded.
According to one aspect of the present invention a method of removing ink and other fine undesirable particles from a slurry comminuted cellulosic fibrous material, containing ink particles and larger contaminants (preferably a slurry containing at least 15% post-consumer recycled pulp, at a consistency of between about 0.5-4.5%, preferably between about 2.5-3.5% by weight, e.g. containing at least 50% post-consumer recycled pulp) which is being fed to a paper machine using a first set of cleaners, is provided. The method comprises the following steps: (a) Cleaning the slurry in the first set of cleaners to produce at least one reject stream containing the majority of the undesirable contaminants including ink particles, and at least one accept stream containing fewer contaminants than the at least one reject stream. (b) Fractionating at least one reject stream to produce a first, fine fraction, stream containing the majority of the ink and other fine undesirable particles, and a second, coarse fraction, stream containing most of the fiber and larger contaminants. (c) Mechanically agitating the second, coarse fraction, stream to disrupt fiber flocs and reduce the size of contaminants to create a third stream containing few or no fiber flocs and no large particles. (d) Returning the third stream containing fiber but few or no ink particles, fiber flocs, or large contaminates, to the first set of cleaners. And, (e) feeding a cleaned stream of slurry from the first set of cleaners to a paper machine.
The first set of cleaners preferably comprises at least three stages of vortex cleaners and step (d) may be practiced by returning the third stream to before the second cleaning stage but upstream of where the reject stream is passed to fractionation. There may be the further steps of removing ink particles from the first stream via flotation or cleaning to produce a fourth stream, and returning the fourth stream to the first set of cleaners, e.g. upstream of where the reject steam is passed to fractionation. Step (b) may be practiced by passing the slurry through screens having slots between 0.001-0.020 inches in width, preferably between 0.006-0.010 inches.
In a preferred embodiment there is an additional step in which the ink-laden first fine-fraction of step (b) is further treated to remove ink particles. This step may comprise flotation for example, dissolved-air flotation, or micro-flotation!, or GSC.RTM. Hydrocyclone flotation as sold by Ahlstrom Machinery, and described in U.S. Pat. Nos. 5,069,751; 5,131,980, or 5,529,190. Another embodiment includes a further cleaning step (f), between steps (c) and (d), in which the third stream is cleaned prior to return to the first set of cleaners, and a diluting step (g) between steps (c) and (f).
The mechanical agitating device used in the practice of step (c) is, for example, a dispersing device, such as an MDR.RTM. disperser as sold by Ahlstrom Machinery, or a deflaking device, such as a deflaker-type pump, or any similar device which agitates the slurry to disrupt fiber flocs and reduce contaminant size and improve contaminant distribution.
In another embodiment of the invention, the first set of cleaners in step (a) includes at least three stages of cleaners, and step (d) is practiced by introducing the third or second stream downstream of the first or second cleaner stage.
According to another aspect of the present invention a system for producing cleaned comminuted cellulosic fibrous material slurry is provided. The system comprises the following components: A first set of cleaners for separating ink particles from a slurry of comminuted cellulosic fibrous material, including a plurality of cleaning stages, at least one inlet, and at least one rejects outlet. A fractionating device having an inlet connected to the at least one rejects outlet from the first set of cleaners, a first, fine fraction, outlet, and a second, coarse fraction, outlet. A mechanical device having an inlet connected to the second outlet for disrupting fiber flocs and reducing the size of contaminants, and having an outlet. And, the mechanical device outlet connected to the at least one inlet of the first set of cleaners.
In a preferred embodiment of this invention, the first set of cleaners includes of at least four stages of cleaning. In another embodiment, the first and second set of cleaners are vortex cleaners, for example RB 200 cleaners as sold by Ahlstrom Machinery of Glens Falls, N.Y. The fractionating device is preferably a static bow screen or a rotary pressure screen, for example a Micra-Screen.TM. or Ahldecker.TM. screen as sold by Ahlstrom Machinery.
Another embodiment includes a flotation device for further removing ink particles from the fine-fraction discharged from the fractionating device. The flotation device is preferably a micro-flotation device, a dissolved-air flotation device, or a GSC.RTM. Hydrocyclone with or with out a separation vessel.
A further embodiment of this invention includes a second set of cleaners located downstream of the agitation device for cleaning the slurry prior to returning it to the first set of cleaners, and a dilution tank before the second set of cleaners. The mechanical (agitating) device is preferably the device described above.
According to another aspect of the present invention a paper machine stock preparation system is provided comprising the following components: An approach system to a paper machine. A first set of cleaners for separating ink particles from a slurry of comminuted cellulosic fibrous material, including a plurality of cleaning stages, at least one inlet, a cleaned slurry outlet, and at least one rejects outlet. The cleaned slurry outlet connected to the approach system of a paper machine. A fractionating device having an inlet connected to the at least one rejects outlet from the first set of cleaners, a first, fine fraction, outlet, and a second, coarse fraction, outlet. A mechanical device having an inlet connected to the second outlet for disrupting fiber flocs and reducing the size of contaminants, and having an outlet. And, the mechanical device outlet connected to the at least one inlet of the first set of cleaners.
The details of the components of the paper machine system are preferably as described above for the system for producing claimed slurry. Throughout the practice of the invention the slurry preferably has a consistency of between about 0.1-3.0%, preferably between about 0.5-1.0%, by weight.
It is the primary object of the present invention to provide maximum fiber recovery while even more effectively cleaning a slurry fed to the approach system of a paper machine. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.